Sampling system for a separation channel

ABSTRACT

A sampling system for providing a sample plug  70  from a fluid sample, comprising: a flow channel through which a flow of a fluid sample is passed; plasma generators  12  and  14;  and a control unit configured to drive the plasma generator  12  at a power level at which the fluid sample is modified by the plasma P 1  and interrupt busy the plasma P 1  for a period of time such as to allow a plug of the fluid sample to pass downstream thereof. The volume of the plug  70  is determined by the time period during which the plasma P 1  is interrupted. With the plasma P 1  being maintained, the components of the plug  70  separate on transit through a separation channel  10.  These components pass through the second plasma P 2,  and the emissions generated are detected by an optical detector  16.

[0001] The present invention relates to a sampling system, preferably asa microfabricated chip-based unit, for and a method of providing avolume, as a sample plug, from a flow of a fluid sample, in particular agaseous sample, and to a measurement system incorporating the samplingsystem.

[0002] Precisely metered volumes of fluid samples, typically very smallvolumes of up to 2 μl, are required by many measurement systems, such asgas chromatographs, for accurate sample analysis.

[0003] Microsyringes are commonly used to deliver metered volumes offluid samples. These syringes, however, have a limited volumetricaccuracy, and as such are not suited to the delivery of very smallvolumes.

[0004] Minaturized chip-based sampling systems have been proposed, butthese systems are complex and require moving components to valve andmeter a fluid sample. As will be appreciated, the fabrication of systemsincluding such minaturized components is particularly difficult, and inrequiring moving parts can suffer from problems of reliability.

[0005] It is thus an aim of the present invention to provide an improvedsampling system for providing sample plugs of small volume from a flowof a fluid sample, and in particular a sampling system which requires nomoving parts. It is also an aim of the present invention to provide animproved sampling method.

[0006] Accordingly, the present invention provides a sampling system forproviding a volume, as a sample plug, from a flow of a fluid sample,comprising: a flow channel through which a flow of a fluid sample is inuse passed; a plasma generator for generating a plasma in the flowchannel; and a control unit operably configured to drive the plasmagenerator at a power level at which the fluid sample passingtherethrough is modified by the plasma and interrupt the plasma for apredeterminable period of time such as to allow a sample plug of thefluid sample to pass downstream thereof.

[0007] Preferably, the control unit is configured to switch the plasmagenerator between a first power level at which the fluid sample ismodified by the plasma and a second, lower power level.

[0008] More preferably, the fluid sample is substantially unmodified bythe plasma at the second power level.

[0009] Still more preferably, the second power level is a zero powerlevel at which the plasma generator is switched off.

[0010] Preferably, the control unit is configured to drive the plasmagenerator at a power at which the fluid sample is destroyed by theplasma.

[0011] In one embodiment the control unit can be configured to switchthe plasma generator between three or more power levels.

[0012] Preferably, the plasma generator includes first and secondelectrodes across which a voltage is applied to generate the plasma.

[0013] The flow channel can be a substantially linear channel or ameandering channel which preferably includes a plurality of bends.

[0014] Preferably, the sampling system further comprises at least onefurther plasma generator upstream of the first-mentioned plasmagenerator, and wherein the control unit is operably configured to driveeach of the plasma generators at a power level at which the fluid samplepassing therethrough is modified by the respective plasma and interruptthe respective plasma for a predeterminable period of time.

[0015] Preferably, the sampling system further comprises a substratechip in which the flow channel and the or each plasma generator aredefined.

[0016] The present invention also extends to a measurement systemcomprising the above-described sampling system.

[0017] Preferably, the flow channel defines a separation channeldownstream of the plasma generator in which the components of the sampleplug are in use separated, and further comprising a detector downstreamof the separation channel for detecting the separated components of thesample plug.

[0018] More preferably, the detector comprises a plasma generator forgenerating a detection plasma in the flow channel downstream of theseparation channel and an optical sensor for detecting the opticalemission of the detection plasma.

[0019] The present invention also provides a method of providing avolume, as a sample plug, from a flow of a fluid sample, comprising thesteps of: providing a sampling system comprising a flow channel and aplasma generator for generating a plasma in the flow channel; passing aflow of a fluid sample through the flow channel; generating a plasma inthe flow channel at a power level to modify the fluid sample passingtherethrough; and interrupting the plasma for a predeterminable periodof time such as to allow a sample plug of the fluid sample to passdownstream thereof.

[0020] Preferably, the step of interrupting the plasma comprises thestep of switching the plasma generator to a second, lower power level.

[0021] More preferably, the fluid sample is substantially unmodified bythe plasma at the second power level.

[0022] Still more preferably, the second power level is a zero powerlevel at which the plasma generator is switched off.

[0023] Preferably, the fluid sample is destroyed by the plasma at thefirst power level.

[0024] In one embodiment the step of interrupting the plasma comprisesthe step of switching the plasma generator between three or more powerlevels.

[0025] Preferably, the plasma generator includes first and secondelectrodes across which a voltage is applied to generate the plasma.

[0026] The flow channel can be a substantially linear channel or ameandering channel which preferably includes a plurality of bends.

[0027] Preferably, the method further comprises the step of generatingat least one further plasma upstream of the first-mentioned plasma at apower level to modify the fluid sample passing therethrough, and whereinthe step of interrupting the plasma comprises the step of interruptingeach of the respective plasmas for a predeterminable period of time.

[0028] Preferably, the sampling system further comprises a substratechip in which the flow channel and the or each plasma generator aredefined.

[0029] Preferably, the fluid sample is a gaseous sample. In this contextthe term gaseous sample is to be understood as encompassing gases andsupercritical fluids.

[0030] A preferred embodiment of the present invention will now bedescribed hereinbelow by way of example only with reference to theaccompanying drawings, in which:

[0031]FIG. 1 schematically illustrates a microfabricated chip-basedmeasurement system in accordance with a preferred embodiment of thepresent invention;

[0032] FIGS. 2(a) to (g) schematically illustrate the operation of themeasurement system of FIG. 1;

[0033] FIGS. 3 to 5 schematically illustrate spectra detected by thedetector of the measurement system of FIG. 1; and

[0034]FIG. 6 illustrates further spectra detected when operating asystem in accordance with any example embodiment.

[0035]FIG. 1 illustrates a microfabricated measurement system inaccordance with a preferred embodiment of the present invention.

[0036] The measurement system comprises a substrate chip 2 whichincludes a main channel 4, in this embodiment a linear channel, whichincludes an inlet port 6 and an outlet port 8 and defines a separationchannel 10 in a central section thereof, a first plasma generator 12 forgenerating a first plasma P1 upstream of the separation channel 10, asecond plasma generator 14 for generating a second plasma P2 downstreamof the separation channel 10, and an optical detector 16 for detectingthe optical emission of the second plasma P2 generated by the secondplasma generator 14. In an alternative embodiment the main channel 4 canbe a meandering channel which preferably includes a plurality of bends.Preferably, the main channel 4 has a width of from about 150 to 300 μm,a depth of from about 10 to 40 μm and a length of more than about 50 cm.

[0037] The first plasma generator 12 comprises first and secondelectrode-housing regions 18, 20 connected to the main channel 4 atopposed sides thereof, and first and second conductive electrode members22, 24, with each of the electrode members 22, 24 comprising anelectrode 26, 28 disposed in a respective one of the electrode-housingregions 18, 20, a contact pad 30, 32 for providing a means of contact toan external power supply, and a lead 34, 36 connecting the electrode 26,28 and the contact pad 30, 32.

[0038] The second plasma generator 14 comprises first and secondelectrode-housing regions 38, 40 connected to the main channel 4 atopposed sides thereof, and first and second conductive electrode members42, 44, with each of the electrode members 42, 44 comprising anelectrode 46, 48 disposed in a respective one of the electrode-housingregions 38, 40, a contact pad 50, 52 for providing a means of contact toan external power supply, and a lead 54, 56 connecting the electrode 46,48 and the contact pad 50, 52.

[0039] Materials suitable for the electrode members 22, 24, 42, 44include gold and tungsten. In this embodiment the electrodes 26, 28, 46,48 are spaced from the longitudinal edges of the main channel 4. It willbe understood, however, that the electrodes 26, 28, 46, 48 can have anyconfiguration which allow plasmas to be generated therebetween. Further,in this embodiment the electrodes 26, 28, 46, 48 are substantiallyplanar elements which extend over one surface of the respectiveelectrode-housing regions 18, 20, 38, 40. In an alternative embodimentthe electrodes 26, 28, 46, 48, in particular that electrode which actsas the cathode, can be hollow. In this modified chip 2, the electrodes26, 28, 46, 48 are each defined by a conductive layer which extends oversubstantially all of the surfaces of the respective electrode-housingregions 18, 20, 38, 40.

[0040] In this embodiment the plasma generators 12, 14 are configured tobe driven by applying a d.c. high voltage across the respective pairs ofelectrodes 26, 28, 46, 48. In a preferred embodiment inductive orpiezoelectric voltage converters are used as the electrical supply toprovide the very small average currents at the relatively high voltagesrequired to drive the plasma generators 12, 14. As will be appreciated,such voltage converters are much more compact than the conventionalelectrical supply arrangement of a high voltage power supply and highimpedance resistors.

[0041] In this embodiment the optical detector 16 comprises a photodiodewhich is mounted to the one plate of the chip 2 adjacent theplasma-generation region of the second plasma generator 14. With regardto the optical properties, atomic and/or molecular emissions can bemeasured, typically the atomic lines or rotation-vibration bands ofmolecules, for example CH, CN, NH, C2, OH, etc.

[0042] The chip 2 is fabricated from two planar substrate plates, inthis embodiment composed of microsheet glass. In a first step, one plateis etched by HF wet etching to form wells which define the main channel4. In a second step, the other plate is etched by HF wet etching todefine trenches corresponding in shape to the electrode members 22, 24,42, 44. In a third step, each of the trenches is filled with a firstlayer of chromium and a second layer of gold to form the electrodemembers 22, 24, 42, 44. In a fourth step, two holes are drilled byultrasonic abrasion into the other plate so as to provide openingsdefining the inlet and outlet ports 6, 8. In a fifth and final step, thetwo plates are bonded together by direct fusion bonding so as to formthe chip 2. In this embodiment the one plate is of smaller dimensionthan the other plate such that the contact pads 30, 32, 50, 52 areexposed.

[0043] The measurement system further comprises a fluid sample deliveryline 58 which includes a metering valve 60 and is connected to the inletport 6 of the main channel 4, in this embodiment by a Swagelok™connector to a fused silica capillary tube bonded to the chip 2, throughwhich a controlled flow of a fluid sample, in this embodiment a gaseoussample, is in use introduced. In another embodiment the fluid samplecould be a liquid.

[0044] The measurement system further comprises a waste line 62 which isconnected to the outlet port 8 of the main channel 4, in this embodimentby a Swagelok™ connector to a fused silica capillary tube bonded to thechip 2, through which the gaseous sample is exhausted to waste.

[0045] The measurement system further comprises a d.c. high voltagepower supply 64 connected to the contact pads 30, 32, 50, 52 of theelectrode members 22, 24, 42, 44 of the plasma generators 12, 14.

[0046] The measurement system further comprises a computer 66 connectedto the optical detector 16, the valve 60 in the fluid sample deliveryline 58 and the high voltage supply 64 such as to regulate the flow rateof the fluid sample through the main channel 4, control the operation ofthe plasma generators 12, 14 and allow for recordal of the opticalemission of the second plasma P2 generated by the second plasmagenerator 14.

[0047] Operation of the measurement system is as follows.

[0048] In a first step, as illustrated in FIG. 2(a), under the controlof the computer 66, the valve 60 in the delivery line 58 is configuredto deliver a controlled flow of a gaseous sample into the main channel4, the first plasma generator 12 is operated at a high power fordeveloping a first plasma P1 sufficient to at least modify the gaseoussample, in this embodiment destroy the gaseous sample by burning, andthe second plasma generator 14 is operated at a power for developing asecond plasma P2 sufficient to generate emission spectra of theseparated components of the gaseous sample as those components passthrough the second plasma P2. As illustrated in FIG. 2(b), as thegaseous sample flow passes through the first plasma P1, that sample isdestroyed by burning such as to leave only residual components, in thisembodiment CO. Prior to the destroyed gaseous sample flow reaching thesecond plasma P2, the optical detector 16 generates a signalrepresentative only of noise. This detected noise signal is illustratedschematically in FIG. 3. As illustrated in FIG. 2(c), the destroyedgaseous sample then passes through the second plasma P2. When thedestroyed gaseous sample passes through the second plasma P2, theoptical detector 16 generates a signal representative of the emissionfrom the residual components, in this embodiment the CO emission, of thedestroyed gaseous sample. This detected signal, which is substantiallyinvariant with time, is illustrated schematically in FIG. 4.

[0049] In a second step, as illustrated in FIG. 2(d), the power supplyto the first plasma generator 12 is interrupted, thereby interruptingthe first plasma P1.

[0050] In a third step, as illustrated in FIG. 2(e), the power supply tothe first plasma generator 12 is restored after the elapse of apredetermined period of time, thereby restoring the first plasma P1 anddestroying the subsequent gaseous sample passing therethrough. Duringthe period in which the first plasma P1 is interrupted, a volume of thegaseous sample passes as a sample plug 70 into the separation channel 10without any modification; the volume of the sample plug 70 beingdetermined by the time period during which the first plasma P1 isinterrupted. As illustrated in FIG. 2(f), with the first plasma P1 beingmaintained, the components C1, C2, C3 of the sample plug 70 thenseparate on transit through the separation channel 10. These separatedcomponents C1, C2, C3 then pass through the second plasma P2, and theoptical emission generated on each of the components C1, C2, C3 passingthrough the second plasma P2 is detected by the optical detector 16.This detected signal is illustrated diagrammatically in FIG. 5.

[0051]FIG. 6 illustrates a number of detected spectral emissions varyingwith time in a manner showing the separation of various components ofthe material flowing through the channel.

[0052] Finally, it will be understood that the present invention hasbeen described in its preferred embodiments and can be modified in manydifferent ways without departing from the scope of the invention asdefined by the appended claims.

[0053] In one alternative embodiment the first plasma generator 12 can,instead of being switched off to interrupt the generation of the firstplasma P1, be switched to a low power level at which the fluid sample issubstantially unaffected. In this way, the delay in the ignition of thefirst plasma generator 12 can be avoided.

[0054] In another alternative embodiment the chip 2 can include aplurality of plasma generators 12 upstream of the separation channel 10.The use of a plurality of plasma generators 12 allows for themodification of the fluid sample where a single plasma generator 12alone could not achieve the necessary modification because, for example,of a power constraint on each plasma generator 12. The use of aplurality of plasma generators 12 which operate under differentoperating conditions, where spaced a predetermined distance apart, alsoallows for a plurality of different sample plugs to be provided inunison, the components of which plugs are separated in the separationcolumn 10 and the signals detected by the optical detector 16 can bedeconvolved. The use of a plurality of, typically two, plasma generators12 further allows for the plasma generators 12 to be switched on and offconsecutively, thereby obviating any problem of slow decay times.

1. A sampling system for providing a volume, as a sample plug, from aflow of a fluid sample, comprising: a flow channel through which a flowof a fluid sample is in use passed; a plasma generator for generating aplasma in the flow channel; and a control unit operably configured todrive the plasma generator at a power level at which the fluid samplepassing therethrough is modified by the plasma and interrupt the plasmafor a predeterminable period of time such as to allow a sample plug ofthe fluid sample to pass downstream thereof.
 2. The sampling system ofclaim 1, wherein the control unit is configured to switch the plasmagenerator between a first power level at which the fluid sample ismodified by the plasma and a second, lower power level.
 3. The samplingsystem of claim 2, wherein the fluid sample is substantially unmodifiedby the plasma at the second power level.
 4. The sampling system of claim3, wherein the second power level is a zero power level at which theplasma generator is switched off.
 5. The sampling system of any ofclaims 1 to 4, wherein the control unit is configured to drive theplasma generator at a power at which the fluid sample is destroyed bythe plasma.
 6. The sampling system of any of claims 1 to 5, wherein theplasma generator includes first and second electrodes across which avoltage is applied to generate the plasma.
 7. The sampling system of anyof claims 1 to 6, wherein the flow channel is a substantially linearchannel or a meandering channel which preferably includes a plurality ofbends.
 8. The sampling system of any of claims 1 to 7, wherein the fluidsample is a gaseous sample.
 9. The sampling system of any of claims 1 to8, further comprising at least one further plasma generator upstream ofthe first-mentioned plasma generator, and wherein the control unit isoperably configured to drive each of the plasma generators at a powerlevel at which the fluid sample passing therethrough is modified by therespective plasma and interrupt the respective plasma for apredeterminable period of time.
 10. The sampling system of any of claims1 to 9, wherein the sampling system further comprises a substrate chipin which the flow channel and the or each plasma generator are defined.11. A measurement system comprising the sampling system of any of claims1 to
 10. 12. The measurement system of claim 11, wherein the flowchannel defines a separation channel downstream of the plasma generatorin which the components of the sample plug are in use separated, andfurther comprising a detector downstream of the separation channel fordetecting the separated components of the sample plug.
 13. Themeasurement system of claim 12, wherein the detector comprises a plasmagenerator for generating a detection plasma in the flow channeldownstream of the separation channel and an optical sensor for detectingthe optical emission of the detection plasma.
 14. A method of providinga volume, as a sample plug, from a flow of a fluid sample, comprisingthe steps of: providing a sampling system comprising a flow channel anda plasma generator for generating a plasma in the flow channel; passinga flow of a fluid sample through the flow channel; generating a plasmain the flow channel at a power level to modify the fluid sample passingtherethrough; and interrupting the plasma for a predeterminable periodof time such as to allow a sample plug of the fluid sample to passdownstream thereof.
 15. The method of claim 14, wherein the step ofinterrupting the plasma comprises the step of switching the plasmagenerator to a second, lower power level.
 16. The method of claim 15,wherein the fluid sample is substantially unmodified by the plasma atthe second power level.
 17. The method of claim 16, wherein the secondpower level is a zero power level at which the plasma generator isswitched off.
 18. The method of any of claims 14 to 17, wherein thefluid sample is destroyed by the plasma at the first power level. 19.The method of any of claims 14 to 18, wherein the plasma generatorincludes first and second electrodes across which a voltage is appliedto generate the plasma.
 20. The method of any of claims 14 to 19,wherein the flow channel is a substantially linear channel or ameandering channel which preferably includes a plurality of bends. 21.The method of any of claims 14 to 20, wherein the fluid sample is agaseous sample.
 22. The method of any of claims 14 to 21, furthercomprising the step of generating at least one further plasma upstreamof the first-mentioned plasma at a power level to modify the fluidsample passing therethrough, and wherein the step of interrupting theplasma comprises the step of interrupting each of the respective plasmasfor a predeterminable period of time.
 23. The method of any of claims 14to 22, wherein the sampling system further comprises a substrate chip inwhich the flow channel and the or each plasma generator are defined.